1. Field
The present invention relates generally to a process for removing dissolved gasses from reactor coolant in a nuclear power plant and more particularly to apparatus for removing dissolved hydrogen and fission gases from the reactor coolant by passing the coolant over a membrane and extracting the gasses by applying a vacuum.
2. Related Art
During pressurized water reactor plant shutdowns, it is a common practice to drain the reactor coolant system to a level below the reactor vessel flange to the mid-plane of the reactor vessel coolant outlet nozzles. That mid-plane coincides with the mid-plane of the connecting “hot leg” piping leading to the steam generators. This drain-down permits inspection, testing and maintenance, during shutdown, of pumps, steam generators, support structures and other primary system components.
During reactor operation, some fission gases, e.g., xenon and krypton, created by the fission reactions occurring in the nuclear fuel, may enter the reactor coolant system and become dissolved in the reactor coolant. Subsequent to shut down, but before refueling and maintenance operations commence, the concentration of radioactive gases and hydrogen must be reduced to avoid excessive radiation exposure to plant maintenance inspection personnel and reduce the likelihood of an explosion due to a potential spark setting off a flammable mixture of air and hydrogen in the containment atmosphere.
Reactor coolant has previously been degassed using a volume control tank connected to the reactor coolant system. Generally, the reactor coolant system primarily includes such nuclear steam supply system components as the reactor vessel, the steam generators, the reactor coolant pumps and the connecting piping. The volume control tank is part of the system known as the chemical and volume control system which operates in the degassing mode by flashing the dissolved hydrogen and radioactive gases out of the reactor coolant and into the vapor space of the volume control tank. An example of such a system could be found in U.S. Pat. No. 4,647,425.
Typically, a relatively small flow of reactor coolant referred to as the letdown flow is diverted from the reactor coolant system and through the chemical and volume control system. This stream is first cooled then purified in a mixed bed demineralizer, filtered to remove dissolved ionic or suspended particulate material and passed to the volume control tank.
U.S. Pat. No. 4,647,425 proposes an improvement to this chemical and volume control system procedure and reduces the time required to effectively degas the reactor coolant. The method proposed by the patent provides for vacuum degassing a reactor coolant system. The method comprises draining down the reactor coolant system to approximately the mid-point of the hot leg and maintaining the reactor coolant system in an unvented condition during the drain-down operation. Any flashed reactor coolant in the primary side of the steam generator is then refluxed. As used in the above mentioned patent, flashed reactor coolant means liquid coolant which flashes into the steam phase as a result of lower ambient pressure. Refluxed means condensed and cooled. The bulk of the reactor coolant as well as the refluxed reactor coolant, are circulated through a residual heat removal system to cool the reactor coolant. A vacuum is drawn on the reactor coolant system to evacuate any gas stripped from the reactor coolant. Preferably, the step of draining the coolant system establishes a partial vacuum in the unvented reactor vessel and reactor coolant system during drain-down. The partial vacuum is sufficient to cause the reactor coolant to boil at the prevailing temperatures in the reactor coolant system whereby the degassing occurs during the drain-down step.
FIG. 1 shows one prior art embodiment of a vacuum degassing system 10 that is currently in use. The letdown flow enters the system at the inlet 12 and is directed to an inlet 14 of a degasifier column vessel 16 where it enters the interior of the vessel through a spray head 18. A vacuum is drawn on the vessel through conduit 20 by the degasifier vacuum pumps 36. Excess reactor coolant which is not evaporated is drawn from the vessel by discharge pumps 22, with pulse dampeners 24 employed to smooth out the pulses generated by the diaphragm discharge pumps 22. The coolant that is drawn through the discharge pumps 22 is exhausted to a holding tank 26 for return to the system or disposal. The water vapor and non-condensable gases that are separated from the coolant in the degasifier column 16 are routed through a demister 28 to remove any entrained coolant and conveyed to a vapor condenser 30 in which it is placed in heat exchange relationship with chilled water that enters and exits the vapor condenser through inlets and outlets 32 and 34. The radioactive gases and hydrogen are then drawn by vacuum pumps 36 to a degasifier separator 38. The separated coolant is then drawn off by the degasifier separator pumps 40 and discharged to the holding tank 26. The radioactive gas and hydrogen are vented from the degasifier separator 38 vapor space to the reactor plant radioactive waste gas system 42. The nitrogen purge line 44 is provided to purge any residual hydrogen and radioactive gases prior to maintenance.
This traditional approach requires significant energy to operate large vacuum pumps, multiple components, e.g., degasifier columns, transfer pumps, separator vessels, interconnecting piping, valves, and instrumentation, and requires significant building space and support systems, e.g., cooling/chilled water. Thus, while these systems have a long track record, further improvement is desired that will simplify the design, reduce the energy required to operate the system, the amount of building space that is required to house the system and reduce the capital and maintenance costs of the system.